An LED semiconductor luminescent device comprises a semiconductor light-emitting diode (LED), a nixie tube, a symbol tube, a *-shaped tube and a dot-matrix display screen (hereinafter referred to as a “matrix tube”), etc. In fact, each light-emitting unit in the nixie tube, the symbol tube, the *-shaped tube and the matrix tube is a light-emitting diode.
The performances of an LED are determined by parameters and some key parameters related to the LED are as follows: (1) Allowable power dissipation Pm, which refers to the maximum value of the product between the forward DC voltage allowed to be applied on both ends of the LED and the passing-through current. If the value is exceeded, the LED becomes hot and damaged. (2) Maximum forward DC current IFm, which refers to the allowable maximum forward DC current. If the value is exceeded, diodes can be damaged. (3) Inverse peak voltage VRm, which refers to the allowable maximum inverse peak voltage. If the value is exceeded, the light-emitting diode can be broken down and damaged. (4) Operating environment (topm), which refers to the range of the environment temperature for normal operation of light-emitting diodes, and if the operating environment is beyond the temperature range, the light-emitting diodes cannot operate properly, with the efficiency greatly reduced. (5) Forward operating current If, which refers to the forward current value when the light-emitting diode operates normally. In actual use, the IF should be selected below 0.6 IFm as required. (6) Forward operating voltage VF: the working voltage given in the parameter list is achieved in the given forward current and usually measured when IF=20 mA. The light-emitting diode has forward operating voltage VF at 1.4˜3V. When the external temperature rises, the VF decreases. (7) V-I characteristics: the relationship between voltage and current of a light-emitting diode is that the forward voltage is just less than a certain value (called as a threshold value), the current becomes extremely low and the diode cannot be lighted up. When the voltage exceeds the value, the forward current rises rapidly along with the voltage, and the light-emitting diode is lighted up.
Currently, at home and abroad, the LED is actuated by means of constant voltage or constant current; no matter under numerical control or simulation control, when a constant-current and constant-voltage LED actuating power supply is equipped to actuate the LED lamp unit, the constant-voltage or constant-current control technology is adopted during operating no matter how great the VF value of the LED lamp unit is changed, the output voltage and current coincide with the rate value of the lamp unit; the VF values are different for different LED lamp units from different manufacturers, especially the VF value consistency is much poorer for the LED lamp units from those of the factories with a less advanced packaging technology, and after lamp beads completely different in the VF value consistency constitute a lamp unit, with the rising of the LED working temperature, the VF value changes and begins to drift and decrease at the impact of the temperature, but the existing LED power supply sources all have a constant voltage or constant current, so it is completely impossible to adjust the output current and voltage according to the serious drift of the VF value when the LED lamp unit is exposed to a high temperature, thus forcing the LED lamp unit to operate under the condition of low VF voltage and constant current. As a result a direct negative impact that the PN-junction temperature of the LED will continue to rise to speed up light depreciation, flickering and life shortening of the LED lamp unit is brought.
For example, in time of constant-current actuating: when the output voltage and current of an LED actuating power supply is 36V and 2.4 A respectively, the output power should be 86 W; when the temperature rises and the VF value falls, the output voltage will then have tiny changes: 30V, 2.4 A, the output power is 72 W, and the LED lamp unit temperature continues rising to cause the continuous deceasing of the VF value due to the constant-current 2.4 A actuating, while the actual luminous flux of the LED lamp unit is greatly reduced due to lowering of the VF value, and light depreciation, flickering and color temperature deviation of the LED lamp beads can be speeded up when a constant current passes through. In time of constant-voltage actuating: when the output voltage and current of the LED actuating power supply is 36V and 2.4 A respectively, then the output power should be 86 W; when the temperature rises and the VF value decreases, the feedback regulation function of the constant-voltage actuator adjusts the output voltage to 36V, the output current rises to 2.6 A instead, and the output power also rises to 93.6 W, the continuous rising of the PN-junction temperature of the LED lamp unit results in continuous decreasing of the VF value. Moreover, at the moment, the feedback regulation constant-voltage function of the constant-voltage circuit causes the current to further rise, and the power continuously is kept out of limits, which results in burning of the LED lamp unit.